Theoretical Study On The Directionality Of Intramolecular Proton Transfer In Excited States Of HBT Derivatives

Excited state intramolecular proton transfer(ESIPT)is one of the most important photophysical processes.Research into ESIPT can help us understand the microscopic mechanism of molecules so that we can design and develop new light-emitting materials.In this paper,we investigate the ESIPT process of two derivatives of 2-(2'hydroxyphenyl)benzothiazole(HBT)molecule using time-dependent density functional theory(DFT)calculations.The main contents are summarized as follows:(1)We use density functional and time-dependent density functional theory methods to prove the feasibility of intramolecular proton transfer of 3-(benzo[d]-thiazole-2-yl)-2-hydroxy-5-methoxybenzaldehyde(BTHMB)towards the excited state of nitrogen atom.By calculating the potential energy curve,the potential barrier of 0.35kcal/mol indicates that the excited proton transfer process towards oxygen atom is also feasible.In addition,we also calculate the molecular front orbital and molecular vibration spectrum.We revise the previous conclusion that the proton transfer process in the excited state of BTHMB can only be carried out in the direction of nitrogen atoms.Our study shows that both directions of proton transfer in the excited state of BTHMB are feasible.Theoretical calculations also confirm that the fluorescence peak generated by the excited-state proton transfer process towards oxygen atoms is at 565nm,not 500nm as previously speculated.(2)We have studied the properties of the excited states of N'-[(1E)-[5-(2,3-dihydro-1,3-benzothiazole-2-yl)-6-oxy-cyclohexane-1,3-diene-1-yl]methylene]-2-hydroxy-benzohydrazine(HBS)in methanol solvent by using DFT/TD-DFT method.Two structures of HBS molecule were optimized,namely,the HBS-N₁ configuration of N₁ atom in proton transfer process and the HBS-N₂ configuration of N₂ atom in proton transfer process.The corresponding excited state structures and potential energy curves were calculated,and the absorption peaks and fluorescence peaks of different structures of HBS molecule were obtained.According to the excitation state energy curve,we confirmed that both HBS-N₁and HBS-N₂ in methanol solvent can undergo the excited state proton transfer process.It was confirmed that the main peak at 549nm and the shoulder peak at 481nm emitted by HBS molecule in methanol solvent came from the keto*structure of HBS-N₂ configuration and the keto*structure of HBS-N₁configuration,respectively.